The present invention relates to a time-sharing drive optical switch array using a ferroelectric liquid crystal and an optical printer using this optical switch array as a print head.
Different from a conventional nematic liquid crystal, the ferroelectric liquid crystal responds to a DC voltage and its response speed is extremely high and is below 1 msec. An optical switch using such a high response speed can be applied to high-speed printer head and display. In the case of driving the optical switch by a static driving method, the same number of drivers and electrodes as the number of the optical switch elements are necessary. Even if the number of optical switch elements increases, a time-sharing driving method can be used as a method for prevention of an increase in the number of drivers. According to the time-sharing driving method, a plurality of signal electrodes and a plurality of scanning electrodes are combined and the optical switch elements arranged at the cross points between the signal electrodes and the scanning electrodes are time-sharingly driven. This method is disclosed in, for example, U.S. Pat. No. 4,548,476 by Kaneko, issued on Oct. 22, 1985. The invention by Kaneko is characterized in that the polarity of the voltage to be applied to the optical switch elements selected through the scanning electrodes and signal electrodes and the polarity of the voltage to be applied to the other optical switch elements are opposite.
In this driving method, all of the optical switch elements arranged on the scanning electrodes which are not selected are fixed to the light shut-off state, so that only the optical switch elements disposed on the scanning electrodes selected by being applied with the selection signals can transmit the light. Therefore, if this driving method is used for the optical printer head, with an increase in the number of time divisions, the operating period of each optical switch element is reduced and the effective light irradiation energy which can be received on the photo reception side decreases. Therefore, the use efficiency of the light source deteriorates and the printing speed is becomes slow, so that the advantageous feature of the ferroelectric liquid crystal having a high response speed is not effectively used.
The time-sharing driving method of the ferroelectric liquid crystal is also disclosed in JP-A No. 59-193427 filed on Apr. 19, 1983 by Canon Inc. Although this method uses the threshold characteristic of the drive voltage of the ferroelectric liquid crystal, in order to realize the threshold value, the thickness of the liquid crystal layer needs to be controlled to about 1 .mu.m. Therefore, the mass production of such a ferroelectric liquid crystal is, in fact, impossible according to the current manufacturing technology.